This invention relates to the field of nondestructive testing. In particular, the invention concerns a method and apparatus for nondestructive testing of a vehicle, especially aircraft.
As an aircraft takes off, the pressure difference between the inside and the outside increases. When the aircraft reaches cruising altitude, the pressure inside the cabin may differ from the pressure outside by 6 psi or more. This pressure difference causes the radius of the fuselage of a typical commercial aircraft to increase by about one-eighth of an inch. Repeated takeoffs and landings cause the skin of the aircraft to stretch and contract many times. Such stretching can lead to metal fatigue, and can cause bonded joints in the fuselage to become unbonded. These conditions of unbonding often elude detection by conventional means. The prior art has known only relatively crude methods for detecting such defects.
The above problem has become especially acute because many commercial airlines have kept their aircraft in service long beyond the aircraft's anticipated economic life. With a large number of older aircraft in the skies, it becomes especially important to provide an accurate and economical means of nondestructively testing for structural defects.
The present invention includes a method and apparatus for economically testing aircraft and other vehicles for defects. In the preferred embodiment, the invention uses a real-time nondestructive testing device incorporating an electronic shearography camera. The method of the present invention applies stress to an aircraft which simulates the actual stress due to repeated expansions and contractions, and enables one to test for defects rapidly and with varying degrees of automation.